1. Field of the Invention
The present invention relates to a flat type nonaqueous secondary cell for high output power applications, the secondary cell having a laminate type armour case applied for lithium ion secondary cells and lithium polymer secondary cells and the like.
2. Background Art
In recent years, hybrid electric vehicles (HEVs) that use a combination of a battery and gasoline have become popular due to environmental problems and others. This hybrid electric system requires a secondary cell that has large capacity and high output power. A nickel-hydrogen cell is mainly used for the power supply for HEVs because of its superiority in safety. Although a lithium ion cell is superior in energy density compared to a nickel-hydrogen cell, the former has a problem regarding safety due to its use of, for example, organic electrolytes. For this reason, various investigations regarding the safety of a lithium cell for HEVs, as well as for the purpose of obtaining high output power and prolongation of its life have continuously been carried out.
As a positive electrode active material for the lithium ion cell, oxides composed of a composite of lithium, cobalt, nickel, and manganese having layered type or spinel type structures are used. A layered type lithium cobalt oxide, which is superior in cell properties and is easy to synthesize, is currently used most often in small appliances such as cellular phones. However, because cobalt is low in its reserve and therefore expensive, layered type lithium nickel cobalt manganese oxide has been considered as its alternative.
On the other hand, high output power and increased safety are required for a large lithium ion cell for HEV. A spinel type lithium manganese oxide which is mainly used has increased safety, but it has poor durability compared to the layered type. Accordingly, a positive electrode active material, which is composed of a mixture of the spinel type structure oxide having increased safety and the layered type structure oxide having high output power and strong durability, is investigated in JP Patent Publication (Kokai) No. 2002-100358. This mixture of the spinel type and layered type lithium manganese oxides employed in this report, however, had a problem in terms of durability of over ten years as required by applications such as for HEVs.
Moreover, investigations not only on electrode materials but on electrolytes have been carried out as well to aim at strong durability. JP Patent Publication (Kokai) No. 2005-5117 investigates addition of a light metal salt to an electrolyte to prevent a decomposition reaction of the electrolyte at the negative electrode and deterioration due to a reaction of precipitated metal lithium and the electrolyte.
The subject of the present invention is to provide a laminate type nonaqueous secondary cell for a hybrid electric vehicle (HEV) with a superior cycle life using a positive electrode active material having as the main component a mixed material of spinel type and layered type lithium manganese oxides.